Optimized detection of unnecessary inter-RAT handover

ABSTRACT

According to an aspect, a network node operating in a first RAN according to a first RAT is the target of an inter-RAT (IRAT) handover. The network node receives a handover request for a user equipment from a cell in a second RAN operating according to a second RAT. After handover of the user equipment to a cell in the first RAN is completed, the network node configures the user equipment to measure one or more frequencies corresponding to the second RAN. Based on measurements reported by the user equipment for the one or more frequencies, the network node identifies one or more detected cells exceeding a measurement threshold, and sends a handover report towards the second RAN. The handover report includes, for at least one detected cell exceeding the measurement threshold, a physical cell identifier for the detected cell and a frequency identifier for the detected cell.

TECHNICAL FIELD

The technology disclosed herein relates generally to wireless communication networks, and more particularly relates to techniques for reducing unnecessary handovers from one radio access technology to another.

BACKGROUND

Wireless phones and other user equipment supporting a fourth-generation (4G) wireless technology such as the Long Term Evolution (LTE) technology, formally known as Evolved Universal Terrestrial Radio Access (E-UTRA), typically also support a 3G technology, such as the Universal Terrestrial Radio Access (UTRA) technology often referred to as Wideband Code-Division Multiple Access (WCDMA). These same devices might also be compatible with 2G networks, such as the Global System for Mobile Communications (GSM)/EDGE Radio Access Network (GERAN).

These networks may be connected to one another and, in some circumstances, may permit a user equipment (UE) to be handed over from one to another. As shown in FIG. 1, the LTE and GERAN/UTRAN architectures are combined by means of interfaces between the core network nodes of each respective technology. See “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access,” 3GPP TS 23.401, ver. 13.0.0 (September 2014), available at www.3gpp.org. These core nodes include, for example, the Mobility Management Entity (MME), the Serving GPRS Support Node (SGSN), the Serving Gateway (SGW), and the Home Subscriber Server (HSS), all of whose functions are well known to those generally familiar with the family of network standards developed by members of the 3^(rd)-Generation Partnership Project (3GPP).

One of the ways for the LTE and GERAN/UTRAN technologies to communicate with each other is via the RAN Information Management (RIM) protocol, which allows transferring of information from LTE to GERAN/UTRAN and vice-versa in a pre-configured manner. The RIM protocol is specified in “3^(rd) Generation Partnership Project; Technical Specification Group GSM/EDGE Radio Access Network; General Packet Radio Service (GPRS); Base Station System (BSS)—Serving GPRS Support Node (SGSN); BSS GPRS Protocol (BSSGP) (Release 12),” 3GPP TS 48.018, v. 12.3.0 (September 2014), also available at www.3gpp.org.

In the current specifications, a specific type of RIM interaction is defined for the purpose of avoiding unnecessary handovers from LTE to GERAN/UTRAN networks. This interaction is known as “Unnecessary IRAT Handover detection.”

FIG. 2 shows a message sequence chart for the Unnecessary Inter Radio Access Technology (IRAT) Handover detection procedure, as per the current standards, where the target RAT is GERAN. A similar message sequence is also valid when the target RAT is UTRAN.

The operations illustrated in FIG. 2 are described in detail in 3GPP TS 48.018 (cited above), 3GPP TS 36.413 (Release 12), 3GPP TS 25.413 (Release 12), and 3GPP TS 48.008 (Release 12), all of which can be found at www.3gpp.org. The illustrated procedure allows the LTE radio access network (RAN) to configure specific measurement criteria and thresholds for a UE that is handed over from LTE to GERAN/UTRAN. More generally, similar procedures may be used when a UE is handed over from a first RAN 204, operating according to a first RAT, to a second RAN 202, operating according to a second RAT.

Referring again to FIG. 2, the measurement configuration sent from the source RAN/RAT (an LTE network, in this case) to the target RAN/RAT (a GERAN/UTRAN network), is captured in the information elements that make up the IRAT Measurement Configuration IE. Details of the IRAT Measurement Configuration IE can be found in 3GPP TS 48.018, 3GPP TS 25.413, and 3GPP TS 48.008; some of those details are illustrated below, in FIGS. 3 and 4. FIG. 3 shows an IRAT Measurement Configuration Information Element (IE). FIG. 4 a structure of an IRAT Measurement Configuration IE as used for handover from LTE to UTRAN. The configuration of FIG. 4 may be sent by LTE to UTRAN/GERAN via the Source BSS to Target BSS Transparent Container IE or Old BSS to New BSS information IE (in case of handover to GERAN; see 3GPP TS 48.018 and 3GPP TS 48.008) or via the Source RNC to Target RNC Transparent Container IE (in case of handover to UTRAN, see 3GPP TS 25.413) within the handover signalling, i.e. as part of the HANDOVER REQUIRED and HANDOVER REQUEST messages generated in LTE (as shown in step 1 and step 2 of FIG. 2).

Upon reception of such configuration, the UTRAN/GERAN will need to configure the UE (handed over from LTE) to perform measurements for a time duration equal to the Measurement Duration IE and over the E-UTRAN frequencies indicated in the E-UTRA Absolute Radio-Frequency Channel Number (E-ARFCN) IE. The LTE cells for which measurements are taken will be recorded by the target UTRAN/GERAN base station if the measurement results are above preconfigured thresholds specified in the REPORTING_THRESHOLD IE or Reference Signal Received Power (RSRP) IE or Reference Signal Receive Quality (RSRQ) IE.

The measurements performed by the UE as a result of the Unnecessary IRAT Handover procedure will trigger the delivery of an HO Report IE from GERAN/UTRAN to E-UTRAN as part of a RIM message (see step 5 of FIG. 2) if the following is satisfied (excerpt from 3GPP TS 25.413 showing the conditions in UTRAN):

-   -   HO Report should be sent if there is either a single source RAT         cell whose measurement results exceed the threshold for the         whole measurement duration, or a group of source RAT cells         together provide such coverage. The cells that exceed the         threshold in the first UE measurement report are included in the         HO Report. If both thresholds are present, the received radio         measurements must exceed both the RSRP and the RSRQ thresholds         in order to satisfy the indicated radio conditions.     -   When the HO Report is sent from RNC at the end of the configured         measurement duration, it shall set the HO Report Type IE to         “Unnecessary HO to another RAT”. If the measurement period         expires due to an inter-RAT handover towards LTE executed within         the configured measurement duration, the RNC shall set the HO         Report Type IE in the HO Report to “Early IRAT Handover”.     -   No HO Report shall be sent in case no E-UTRAN cell could be         included, or if the indicated period of time is interrupted by         an inter-RAT handover to a RAT different than LTE or by an         intra-UMTS handover with SRNC relocation.

As can be seen from the quote above, the HO Report IE will be generated only if there are detected cells that satisfy the measurement configuration criteria. Cells can be included in the HO Report only if reported UE measurements for each of the cells detected satisfy the configured thresholds for the whole duration of the configured measurement window or for part of such duration, in the event that the measurement window time is interrupted by an inter-RAT handover towards LTE.

In case all the conditions are satisfied, the HO Report IE sent from UTRAN/GERAN to LTE via RIM is constructed as shown in FIG. 5 (see 3GPP TS 36.413). In the HO Report IE, the cells reported in the Candidate Cell List IE are those LTE cells providing good enough coverage, namely fulfilling the criteria specified in the IRAT Measurement Configuration IE (see FIGS. 3 and 4). Such cells are represented by a list of E-UTRAN Cell Global Identities (E-CGIs). The latter can be seen from the specifications of the Candidate Cell ID IE, which is detailed as shown in FIGS. 6 and 7 (see 3GPP TS 36.413).

The discussion presented herein generally assumes a management system having an arrangement like that shown in FIG. 8. In this arrangement, node elements (NE), also referred to as eNodeBs, are managed by a domain manager (DM), also referred to as the operation and support system (OSS). A DM may further be managed by a network manager (NM). The system composed by the DM and NM may be referred as the Operation and Maintenance System (OAM). Two NEs are interfaced by X2, whereas the interface between two DMs is referred to as Itf-P2P. It is further assumed, in the discussion that follows, that a function that automatically optimizes NE parameters can, in principle, execute in the NE, DM, or NM.

SUMMARY

To unambiguously identify measured cells, the network node in a target RAT providing a Handover Report to the source RAT according to the techniques discussed above can use a global cell identifier (such as the E-UTRAN Cell Global Identifier or ECGI) that could identify each cell. However, this would require that each potential target node (e.g., each base station) be provided in advance with information sufficient to identify the cell from the non-global information that is typically available from monitoring the cell itself, such as the Physical Cell Identifier (PCI). This pre-configuration of all target nodes with this information is undesirable.

Embodiments of the presently disclosed techniques and apparatus address this and other problems. In some embodiments, in the case that the target Inter RAT node is not configured with the mapping between PCI and frequency and the cell's ECGI, given that a UE both in UTRAN and in GERAN reports a PCI and a frequency indication for a detected LTE cell, then both the PCI and the detected cell's frequency should be included in an opportune list in the HO Report IE, to assist the target eNB in unequivocally identifying the cell that was detected and reported by the UE. The embodiments described below provide for adding PCI and frequency information for cells fulfilling the Unnecessary IRAT Handover Detection criteria to the existing HO Report IE.

To further specify the measurements expected from the Unnecessary IRAT Handover Detection procedure, and by that potentially limit the number of measurements performed by the UE while served in GERAN, some extra information related to each individual E-UTRAN frequency in the IRAT Measurement Configuration IE could be supplied by the source eNB. This extra information could, for, example consist of an indicator for each individual E-UTRAN frequency, informing the BSS whether the UE (handed over from LTE) shall be configured for measurements of (1) E-UTRAN macro neighbor cells only, or (2) E-UTRAN CSG cells only, or (3) possibly both cell types for the indicated E-UTRAN frequency. As an alternative to a frequency-specific indicator, a common indicator valid for all E-UTRAN frequencies included in the IRAT Measurement Configuration IE could be supplied by the source eNB.

An example method according to some embodiments is suitable for implementation in a network node operating in a first RAN according to a first RAT. In this case, the network node is the target of an IRAT handover. The method includes receiving a handover request for a user equipment from a cell in a second RAN operating according to a second RAT and, after handover of the user equipment to a cell in the first RAN is completed, configuring the user equipment to measure one or more frequencies corresponding to the second RAN. Based on measurements reported by the user equipment for the one or more frequencies, the network node identifies one or more detected cells exceeding a measurement threshold, and sends a handover report towards the second RAN. The handover report includes, for at least one detected cell exceeding the measurement threshold, a physical cell identifier for the detected cell and a frequency identifier for the detected cell.

In some embodiments, the network node receives information identifying the one or more frequencies corresponding to the second RAN in a message associated with the handover request. In some of these embodiments, the information identifying the one or more frequencies comprises an E-UTRA Absolute Radio-Frequency Channel Number (EARFCN). In some embodiments, the network node also receives, in a message associated with the handover request, measurement information indicating, for at least one of the one or more frequencies, whether or not the user equipment should measure closed-subscriber-group (CSG) cells corresponding to the at least one of the one or more frequencies. In these embodiments, configuring the user equipment to measure one or more frequencies corresponding to the second RAN comprises configuring the user equipment to measure CSG cells or not to measure CSG cells, according to the received measurement information. The measurement indicator in some of these embodiments may be a single indicator indicating whether or not the user equipment should measure CSG cells for all of the one or more frequencies. In others of these embodiments, a separate indicator is provided for each of the one or more frequencies.

In some embodiments, the handover report includes, for at least one of the detected cells exceeding the measurement threshold, a global cell identifier. In some embodiments, a physical cell identifier and frequency identifier is included only for those detected cells for which a global cell identifier is not known or could not be derived from the measurements reported by the user equipment. As shown by the embodiments, the first and second RATs may be different RATs.

Another example method according to the present techniques is carried out in a network node operating in a first RAN according to a first RAT. In this case, the network node is the source node of an IRAT handover. The example method includes initiating a handover of a user equipment from a cell in the first RAN to a cell in a second RAN, operating according to a second RAT, by sending a handover-required indication towards the second RAN. The example method includes, after handover of the user equipment to the cell in the second RAN is completed, receiving a handover report from the second RAN. The handover report comprises, for at least one cell detected by the user equipment, a physical cell identifier for the detected cell and a frequency identifier for the detected cell. The network node then identifies a global cell identifier for the at least one cell, based on the physical cell identifier and frequency identifier, and adjusts one or more mobility settings with respect to the at least one cell, in response to receiving the handover report.

In some embodiments, the method further comprises sending, towards the second RAN, information identifying one or more frequencies to be measured by the user equipment, in a message associated with the handover request. The information identifying the one or more frequencies may comprise an EARFCN, in some embodiments. In some embodiments, the network node still further sends, in a message associated with the handover request, measurement information indicating, for at least one of the one or more frequencies, whether or not the user equipment should measure CSG cells corresponding to the at least one of the one or more frequencies. This measurement information may include a single indicator indicating whether or not the user equipment should measure CSG cells for all of the one or more frequencies, in some embodiments. In others, the measurement information comprises a separate indicator for each of the one or more frequencies.

In some embodiments, the network node sends information identifying a measurement threshold in a message associated with the handover request. In some embodiments, the received handover report includes, for at least one cell detected by the user equipment, a global cell identifier.

The techniques summarized above and detailed below allow a drastic reduction of the configuration effort needed to store details about potential LTE neighbor cells on GERAN BSSs and UTRAN Radio Network Controllers (RNCs). The techniques also allow a reduction of IRAT System Information Broadcast (SIB) reading measurements for UEs in UTRAN, during the procedures of Unnecessary IRAT Handover Detection.

The methods outlined herein allow a GERAN BSS or an UTRAN RNC to discover new LTE neighboring cells that are reliable handover candidates and to trigger configuration/retrieval of information for such cells, so that procedures towards these cells (e.g., handovers) can be started when needed. Some of the methods detailed herein can also potentially limit the number of measurements performed by a UE in GERAN.

In the detailed description that follows, the several embodiments summarized above are described in detail, and descriptions of corresponding apparatus for carrying out the methods summarized above are described. It should be appreciated however, that these embodiments are intended to be illustrative, and not exhaustive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of combined LTE and GERAN/UTRAN architectures.

FIG. 2 is a message sequence chart for the Unnecessary IRAT Handover detection procedure, as per the current standards.

FIG. 3 illustrates the structure of an IRAT Measurement Configuration IE.

FIG. 4 illustrates the structure of an IRAT Measurement Configuration IE as used for handover from LTE to UTRAN.

FIG. 5 illustrates the structure of the HO Report IE message as per TS 36.413.

FIG. 6 illustrates the structure of an IRAT Cell ID.

FIG. 7 illustrates the structure of an E-UTRAN CGI (ECGI) IE.

FIG. 8 is a diagram of a management system.

FIG. 9 is a block diagram of a network node configured to send HO Reports, according to some embodiments.

FIG. 10 illustrates the structure of an HO Report with a Candidate PCI IE, according to some embodiments.

FIG. 11 illustrates a range bound for Candidate PCIs, according to some embodiments.

FIG. 12 illustrates the structure of a Candidate PCI IE, according to some embodiments.

FIG. 13 is a message sequence chart for enhancement of a HO Report with PCI and frequency identifier of suitable LTE cells, according to some embodiments.

FIG. 14 is a flowchart illustrating a method for sending HO Reports, according to some embodiments.

FIG. 15 is a flowchart illustrating a method for adjusting mobility settings in response to a HO Report, according to some embodiments.

FIG. 16 is a block diagram of a functional implementation of a network node for sending HO Reports, according to some embodiments.

FIG. 17 is a block diagram of a functional implementation of a network node for adjusting mobility settings in response to a HO Report, according to some embodiments.

DETAILED DESCRIPTION

Inventive concepts will now be described more fully hereinafter with reference to the accompanying drawings, in which examples of embodiments of inventive concepts are shown. These inventive concepts may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and fully convey the scope of present inventive concepts to those skilled in the art. It should also be noted that these embodiments are not mutually exclusive. Components from one embodiment may be tacitly assumed to be present or used in another embodiment.

For purposes of illustration and explanation only, embodiments of the present inventive concepts are described herein in the context of operating in or in association with a RAN that communicates over radio communication channels with mobile terminals, also interchangeably referred to as wireless terminals or UEs, using a particular RAT. More specifically, embodiments are described in the context of the E-UTRAN, sometimes referred to as the Evolved UMTS Terrestrial Radio Access Network and widely known as the LTE system. However, it will be appreciated that the techniques may be applied to other wireless networks, as well as to successors of the E-UTRAN. Thus, references herein to signals using terminology from the 3GPP standards for LTE should be understood to apply more generally to signals having similar characteristics and/or purposes, in other networks.

As used herein, the terms “mobile terminal,” “wireless terminal,” “user equipment,” or “UE” may be used to refer to any device that receives data from and transmits data to a communication network, any of which may be, for example, a mobile telephone (“cellular” telephone), laptop/portable computer, pocket computer, hand-held computer, desktop computer, a machine to machine (M2M) or machine-type communication (MTC) device, a sensor with a wireless communication interface, etc. Devices of any of these types may be adapted, according to known techniques and according to the additional techniques disclosed herein, for operation in a device-to-device (D2D) mode, where such operation may include the transmitting and receiving of certain signals that are similar to or identical with corresponding signals used when operating within a cellular network, i.e., in a device-to-base-station operating mode.

In some embodiments, the term “network node” is used. This term may refer to any type of radio network node or other node in the fixed portion of a wireless communications network, which communicates with a UE and/or with another network node. Examples of network nodes are an eNodeB or eNB, a base station (BS), multi-standard radio (MSR) radio node such as MSR BS, network controller, radio network controller (RNC), base station controller (BSC), relay, donor node controlling relay, base transceiver station (BTS), access point (AP), transmission points, transmission nodes, remote radio unit (RRU), remote radio head (RRH), nodes in distributed antenna system (DAS), core network node (e.g., mobile switching center or MSC, MME etc.), O&M, OSS, self-organizing networks (SON), positioning node (e.g., Evolved Serving Mobile Location Centre, or E-SMLC), minimization of drive-testing (MDT), etc.

Several of the techniques and methods described herein are implemented using electronic data processing circuitry and other electronic hardware provided in a network node. In some cases, the network node is a base station, and thus further includes radio circuitry for communicating with one or more user equipments.

For example, FIG. 9 shows an example network node, in this case a base station 10 (for example an LTE eNodeB or a GERAN BSS), that can be used in some of the example embodiments described herein. It will be appreciated that although a macro eNB will not, in practice, be identical in size and structure to a small cell eNB, for the purposes of illustration, the base stations 10 are assumed to include similar components. Thus, whether or not base station 10 corresponds to a macro base station or a small cell base station, it comprises a processing circuit that controls the operation of the base station 10. The processing module 40, which may include one or more microprocessors, microcontrollers, digital signal processors, specialized digital logic, etc., is connected to a transceiver module 42 with associated antenna(s) 44 that are used to transmit signals to, and receive signals from, UEs in the network. The base station 10 also comprises a memory circuit 46 that is connected to the processing module 40 and that stores program and other information and data required for the operation of the base station 10. Together, the processing module 40 and memory circuit 46 may be referred to as a “processing circuit,” and are adapted, in various embodiments, to carry out one or more of the network-based techniques described below.

Base station 10 also includes components and/or circuitry, such as eNodeB interface 48, for allowing the base station 10 to exchange information with other base stations 10 (for example, via an X2 interface) and components and/or circuitry, such as core network interface 49, for allowing the base station 10 to exchange information with nodes in the core network (for example, via the S1 interface). It will be appreciated that base stations for use in other types of networks (e.g., UTRAN or WCDMA RAN) will include similar components to those shown in FIG. 9 and appropriate interface circuitry 48, 49 for enabling communications with the other network nodes in those types of networks (e.g., other base stations, mobility management nodes and/or nodes in the core network).

In a working group meeting, the RAN3 working group of 3GPP agreed to a document, “Response LS on the routing information for the unnecessary handover to another RAT detection,” 3GPP doc. R3-142602. The document states, “In order to avoid configuring in the BSS for each and every involved ECGI the corresponding Global eNB ID, RAN3 decided at RAN3#85bis that the source Global eNB ID should therefore also be conveyed in addition to the E-CGI and TAI.” The above statement may be interpreted to say that signaling of the eNB ID (of the eNB requesting Unnecessary IRAT Handover Detection) as part of the HO Required message from source eNB to target GERAN BSS is sufficient to avoid configuring the BSS with ECGI information. The same can be said of a UTRAN RNC target. However, this assumed advantage of avoiding configuration efforts in the target BSS or target RNC is only partially valid, because knowledge of the eNB ID only avoids any need to configure the target IRAT node (the target Inter-RAT node, i.e., the node receiving handover of a UE from another RAT) of the source eNB's Global eNB ID. Namely, the target IRAT node would not need any configured parameters concerning the source eNB's cell, towards which the HO Report IE should be addressed as part of the Unnecessary IRAT Handover response. This is because the S1: HO Required message triggering Unnecessary IRAT Handover Detection would then already include the E-CGI, Tracking Area Identity (TAI), and eNB ID of the cell from which the function was initiated and where the HO Report should be routed.

However, the HO Report, constructed as shown in FIG. 5 and as explained above, includes a candidate cell list, which contains E-CGIs of cells detected by the UE while in the target RAT. Therefore, the problem of configuring a target RAT node (e.g., a GERAN BSS) with neighbor LTE cell information still applies, because the node that has to send the HO Report IE to the source eNB must be able to map the measurements provided by the UE (fulfilling the measurement configuration criteria as provided in the IRAT Measurement Configuration IE) to an E-CGI of the detected cell. This has to be done for all the reported cells satisfying the Unnecessary IRAT Handover detection criteria.

The problem can be better understood if it is considered that a UE in GERAN reports only the PCI of the detected target LTE cell, as well as an index identifying the LTE cell's frequency, such index mapping to a list of frequencies broadcast by the serving BSS to all UEs. Likewise, a UE in UTRAN would first report the PCI and target frequency of the LTE cell detected. It may be possible in UTRAN to trigger further measurements for the UE to acquire more information about the detected cell, but such extra measurements would incur long data traffic interruptions and battery consumption. Therefore, it is desirable that the UE limits LTE cell measurements to PCIs and frequencies.

For these reasons, if the E-CGI for all the LTE cells reported by the UE and fulfilling the Unnecessary IRAT HO Detection criteria must be included in the HO Report IE, then the target GERAN BSS must be configured with mapping between all of the reported PCIs and frequencies and ECGIs to be included in the HO Report IE. Likewise, if it is desired to avoid extra inter-RAT measurements, a target UTRAN RNC would need to be configured in a similar way.

The techniques and apparatus described below enable the possibility of reporting all LTE cells fulfilling the Unnecessary IRAT HO Detection criteria in the HO Report IE without the need for configuring the GERAN BSS or UTRAN RNC with global cell identifier information for all neighbor cells. Further, these techniques allow the acquisition of extra information for detected cells that fulfill the Unnecessary IRAT Handover Detection criteria.

Another problem with the current standards arises from the fact that, when requesting measurements from a UE being served in GERAN, the BSS provides the E-UTRAN frequencies for macro neighbor cells in one optional IE and the E-UTRAN frequencies for the Closed Subscriber Group (CSG) neighbor cells in another optional IE in the measurement request message. An example of such a message is the PACKET MEASUREMENT ORDER message, sent on the Packet Associated Control Channel (PACCH) from the GERAN BSS to the UE, containing E-UTRAN frequencies for macro neighbor cells in the E-UTRAN Parameters IE and E-UTRAN frequencies for CSG neighbor cells in the E-UTRAN CSG Description IE (a condition for the BSS to send the E-UTRAN CSG Description IE to the UE is that the UE has indicated support for E-UTRAN CSG Cells Reporting in its capabilities). The implication of excluding the E-UTRAN CSG Description IE from the PACKET MEASUREMENT ORDER message is that the UE will not perform any measurements on E-UTRAN CSG cells, thus the UE will not send any measurement reports to the network for this type of cells.

As described above, the IRAT Measurement Configuration IE contains a list of E-UTRAN frequencies indicated by the EARFCN IE for which the target GERAN BSS will configure the UE to perform measurements. However, the IRAT Measurement Configuration IE does not contain any information that would help the GERAN BSS to conclude whether it shall configure the UE to perform (and report) measurements on E-UTRAN CSG neighbor cells for the indicated E-UTRAN frequencies. This becomes more evident when cells are not configured in the BSS for one or more of the indicated E-UTRAN frequencies.

As a result of this, and depending on BSS implementation, E-UTRAN CSG neighbor cells may potentially be excluded when requesting measurements from the UE, hence only measurement results from E-UTRAN macro neighbor cells may be included in the HO Report IE although the intention when sending the Unnecessary IRAT Handover Detection measurement request from the source eNB was to also cover E-UTRAN CSG cells in the measurements.

It is worth noting that measurements of CSG cells are rather costly for the UE while being served in GERAN, due to the size of the routing information included in the measurement report, hence it is most likely that some BSS (again depending on implementation) will only request a UE to perform measurements on CSG cells when configured to do so.

In a similar way, a BSS that requests a UE to perform measurements on E-UTRAN macro neighbor cells as well as on E-UTRAN CSG neighbor cells for all of the E-UTRAN frequencies included in the IRAT Measurement Configuration IE, may cause an unnecessary load to the UE for the reason indicated above.

A way to obviate the first of the problems described above is to allow reporting of extra information in the HO Report IE. In particular, such extra information should not require configuration of information regarding neighbor LTE cells in the inter-RAT target node.

One proposal to include a list of detected Physical Cell Identities (PCIs) to the HO Report IE was already briefly described in a way-forward document submitted at a 3GPP working group meeting. See “Way Forward on GERAN LS on routing information for unnecessary handover,” 3GPP doc. R3-142539. However, this enhancement would not be sufficient, because a PCI may be reused across different frequencies. As a result, if only a list of detected and suitable (for unnecessary IRAT Handover Detection purposes) PCIs is provided in the HO Report IE, and in the event that one or more of these PCIs are re-used across different carriers, the receiving node would not know to which cells the PCIs correspond, and would not be able to apply mobility parameters changes to prevent the unnecessary IRAT handover in a reliable way.

It is worth noting that such mobility parameter changes may consist of prioritizing one of the reported LTE cells as a handover target, in replacement of the inter-RAT handover target previously selected. Therefore, applying such adjustments to the wrong cell (wrongly identified due to reuse of the reported PCIs across different carrier frequencies) may lead to major mobility problems and to performance degradation.

A better solution, according to some embodiments, includes for the case when the target inter-RAT node is not configured with the mapping between PCI and frequency and the cell's ECGI, given that a UE both in UTRAN and in GERAN reports a PCI and a frequency indication for a detected LTE cell, then both the PCI and the detected cell's frequency should be included in an opportune list (or candidate list) in the HO Report IE, to assist the target eNB in unequivocally identifying the cell that was detected and reported by the UE. The following discussion thus includes details on how to add PCI and frequency information for cells fulfilling the Unnecessary IRAT Handover Detection criteria to the existing HO Report IE.

To further specify the measurements expected from the Unnecessary IRAT Handover Detection procedure, and by that potentially limit the number of measurements performed by the UE while served in GERAN, some extra information related to each individual E-UTRAN frequency in the IRAT Measurement Configuration IE could be supplied by the source eNB. This extra information could, for, example consist of an indicator for each individual E-UTRAN frequency, informing the BSS whether the UE (handed over from LTE) shall be configured for measurements of (1) E-UTRAN macro neighbor cells only, or (2) E-UTRAN CSG cells only, or (3) possibly both cell types for the indicated E-UTRAN frequency. As an alternative to a frequency-specific indicator, a common indicator valid for all E-UTRAN frequencies included in the IRAT Measurement Configuration IE could be supplied by the source eNB.

Thus, according to one aspect of the solutions detailed herein, a node requested to configure and collect measurement for the purpose of Unnecessary IRAT Handover Detection includes, in the report of cells fulfilling the detection criteria, cells for which cell configuration parameters such as the ECGI are not available. Instead of the EGCIs for these cells, a list of PCIs and frequency indicators for the detected cells is included in the Unnecessary IRAT Handover Detection report, named HO Report IE.

In some embodiments of the presently disclosed techniques, a list of PCIs and frequency identifiers such as the EUTRA Absolute Radio Frequency Channel Number (EARFCN) is added to the current HO Report IE sent over a RIM message as part of the SON Transfer Request Container specified in 3GPP TS 36.413. Each PCI included in such a list corresponds to a cell, detected and reported by the UE configured with measurements for the Unnecessary Inter RAT Handover Detection function, which has fulfilled the measurement conditions specified in the measurement configuration sent to the target IRAT node via handover signaling. As noted above, each PCI included in the list by the target IRAT node may not have a corresponding ECGI available at the target IRAT node. In some embodiments, the PCI and frequency identifier are included only if the corresponding ECGI has not been added in the existing Candidate Cell List IE of the HO Report IE.

One possibility for the frequency identifier is the EARFCN, which points at the center frequency of an LTE carrier. Such parameter can be also replaced by an indication of a carrier frequency band or frequency band on which the UE in the target IRAT node was configured to perform measurements that brought to the discovery of the corresponding PCI. For the sake of simplicity the term “frequency identifier” will be used to specify any parameter such as an EARFCN, or a carrier frequency or a cell frequency or the frequency on which the UE in the target IRAT node was configured to perform the measurements that lead to the detection of the corresponding PCI.

According to this approach, the PCI and frequency identifier are sent together in the HO Report IE if a detected cell fulfills the Unnecessary IRAT Handover Detection criteria and if the target IRAT node cannot deduce a corresponding ECGI. In some embodiments, a way to amend the current specifications to achieve this is shown in FIGS. 10 and 11, where a Candidate PCI List is added to the structure of the HO Report IE. FIG. 12 shows an implementation of the added Candidate PCI List added to the 3GPP standards, which contains the PCI and the EARFCN of the detected cell.

As can be seen from the example in FIG. 10, the semantics description of the Candidate PCI IE explains that the presence of cells reported by means of PCI and EARFCN (or indeed any of the Frequency Identifiers described above) excludes the presence of the same cells in the existing Candidate Cell List IE. Indeed, if for such cells the ECGI can be derived by the target RAT node, the cell shall be reported by means of its ECGI in the Candidate Cell List IE. The latter would avoid any confusion at the node receiving the HO Report IE, such confusion for example consisting of receiving the same cell twice, once in the Candidate Cell List IE and another time in the Candidate PCI List IE. The latter might lead the receiving node to understand that the cell reported by means of its PCI is not configured (e.g., is not stored as a neighbor) in the target IRAT node and as a consequence mechanisms may be triggered to ensure that the cell is configured at the target IRAT node.

However, in an alternative approach, it would be possible for the same cell to be identified in both the Candidate Cell List IE and the Candidate PCI List IE, provided that the receiving node understands that the cell reported in both lists is configured at the target IRAT node.

To control the amount and type of information provided in the HO Report IE as per previous embodiments, some embodiments of the techniques described above provide for an indication to be provided from the source eNB to the target IRAT node, the indication indicating whether or not cells transmitting a CSG ID should be included in the Unnecessary IRAT Handover Detection and reporting mechanism. In various embodiments or instances, they could be included together with other cells or included alone, i.e. only cells transmitting a CSG ID—closed, hybrid, or both—could be included, or excluded. Such indication may comprise information on whether to include CSG cells, namely cells that can only be accessible by UEs that are member of the CSG ID; hybrid cells, i.e., cells provided with a CSG ID and accessible to all UEs but where UEs that are member of the CSG ID may be treated in a prioritized way, or both closed and hybrid cells. The indication may indicate whether to include closed cells and/or hybrid cells alone or whether to include closed cells and/or hybrid cells with other cells. The indication could be provided as part of the IRAT Measurement Configuration IE as discussed above, namely for both the cases of target GERAN RAT and target UTRAN RAT. The indication may be provided on a per EARFCN indicated by source eNB for Unnecessary IRAT Handover Detection, or as a general indication for all the EARFCNs indicated in the IRAT Measurement Configuration IE. With such indication, the target RAT node may understand whether to collect measurements and run the unnecessary IRAT Handover detection for CSG-type cells (i.e. closed or hybrid cells) in addition to (or instead of) non-CSG-type cells.

FIG. 13 illustrates an example message flow for the Unnecessary IRAT Handover Detection procedure as modified by the techniques described herein. In this non-limiting example, the base station of the first RAN 204 is an LTE eNodeB, and decides to handover a UE to another cell. The target cell is served by the base station of the target RAN 202, which is a GERAN BSS. In step 1, the eNodeB of the first RAN 204 indicates to the core network that a handover is required for a UE in the cell of the first RAN 204. The HANDOVER REQUIRED message may include an IRAT Measurement Configuration IE, as shown in FIG. 4. In step 2, the core network sends a HANDOVER REQUEST message to the GERAN BSS of the target RAN 202. Steps 3 and 4 show a HANDOVER REQUEST ACK and a HANDOVER COMMAND.

At this point, the GERAN BSS configures the UE with Unnecessary IRAT Handover measurements. Detected cells with measurements that exceed measurement thresholds are identified. If the GERAN BSS determines that an ECGI is available for a cell, it is included in the Candidate Cell List IE of a HO Report IE to be sent to the eNodeB. If the GERAN BSS determines that an ECGI is not available, a PCI and Frequency Identifier are included in the Candidate PCI List IE of the HO Report IE. The Frequency Identifier can be a EARFCN.

At step 5, a RIM Request transfer is sent to the eNodeB and may include the HO Report IE. The eNodeB identifies cells by the ECGI in the Candidate Cell List IE. The eNodeB also identifies cells that do not have ECGIs in the Candidate Cell List IE by the PCI and EARFCN in the Candidate PCI List IE. In some cases, this may include determining the ECGI for such a cell from the PCI and EARFCN. The eNodeB then adjusts mobility settings towards the target cell of the target RAN 202 or to detected cells reported in the HO Report IE, which has been enhanced in this embodiment by the Candidate PCI List IE.

Given the above detailed examples of techniques for improving the Unnecessary IRAT Handover detection process, it should be appreciated that these techniques may be applied more generally. FIG. 14 illustrates an example method 1400 that may be carried out by a network node operating in a first RAN 202 according to a first RAT. In this case, the network node is a base station, such as base station 10, for the cell targeted in an IRAT handover.

As shown at blocks 1410 and 1440, the method 1400 includes receiving a handover request for a user equipment from a cell in the second RAN 204 operating according to a second RAT and, after handover of the user equipment to a cell in the first RAN 202 is completed, configuring the user equipment to measure one or more frequencies corresponding to the second RAN 204. The method 1400 includes, based on measurements reported by the user equipment for the one or more frequencies for which the user equipment was configured to measure, identifying one or more detected cells exceeding a measurement threshold, as shown at block 1450, and sending a handover report towards the second RAN 204, as shown at block 1460. The handover report comprises, for at least one detected cell exceeding the measurement threshold, a physical cell identifier for the detected cell and a frequency identifier for the detected cell.

In some embodiments, the network node receives information identifying the one or more frequencies corresponding to the second RAN 204 in a message associated with the handover request. This is shown at block 1420. In some of these embodiments, the information identifying the one or more frequencies comprises an EARFCN. In further embodiments, the network node also receives, in a message associated with the handover request, measurement information indicating, for at least one of the one or more frequencies, whether or not the user equipment should measure CSG cells corresponding to the at least one of the one or more frequencies. This is shown at block 1430. In these embodiments, configuring the user equipment to measure one or more frequencies corresponding to the second RAN 204 comprises configuring the user equipment to measure CSG cells or not to measure CSG cells, according to the received measurement information. The measurement indicator in some of these embodiments may be a single indicator indicating whether or not the user equipment should measure CSG cells for all of the one or more frequencies. In others of these embodiments, a separate indicator is provided for each of the one or more frequencies.

In some embodiments, the handover report includes, for at least one of the detected cells exceeding the measurement threshold, a global cell identifier. In some embodiments, a physical cell identifier and frequency identifier is included only for those detected cells for which a global cell identifier is not known or could not be derived from the measurements reported by the user equipment.

FIG. 15 illustrates another example method 1500 according to the present techniques, again carried out in a network node, such as base station 10, operating in a first RAN 204 according to a first RAT. In this case, however, the network node is the source node of an IRAT handover, rather than the target node as in FIG. 14. Thus, the “first” RAN and “first” RAT in this case may differ from the first RAN and first RAT discussed in connection with FIG. 14. For example, the first RAN in FIG. 14 was RAN 202. In FIG. 15, the first RAN is RAN 204.

As shown at block 1510, the example method 1500 includes initiating a handover of a user equipment from a cell in the first RAN 204 to a cell in the second RAN 202. This may be done by sending a handover-required indication towards the second RAN 202. After handover of the user equipment to the cell in the second RAN 202 is completed, the network node receives a handover report from the second RAN 202, as shown at block 1520. The handover report comprises, for at least one cell detected by the user equipment, a physical cell identifier for the detected cell and a frequency identifier for the detected cell. As shown at block 1530, the network node then identifies a global cell identifier for the at least one cell, based on the physical cell identifier and frequency identifier. Finally, as shown at block 1540, the network node adjusts one or more mobility settings with respect to the cell in the second RAN 202 and/or with respect to the at least one cell detected by the user equipment, in response to receiving the handover report.

In some embodiments, the method further comprises sending, towards the second RAN 202, information identifying one or more frequencies to be measured by the user equipment, in a message associated with the handover-required indication. (This may be same message as the handover request, in some embodiments). The information identifying the one or more frequencies may comprise an EARFCN. In some embodiments, the network node still further sends, in a message associated with the handover-required indication, measurement information indicating, for at least one of the one or more frequencies, whether or not the user equipment should measure CSG cells corresponding to the at least one of the one or more frequencies. This measurement information may include a single indicator indicating whether or not the user equipment should measure CSG cells for all of the one or more frequencies, in some embodiments. In others, the measurement information comprises a separate indicator for each of the one or more frequencies.

In some embodiments, the network node sends information identifying a measurement threshold in a message associated with the handover-required indication. In some embodiments, the received handover report includes, for at least one cell detected by the user equipment, a global cell identifier.

Embodiments of the presently disclosed techniques include the several methods described above, including the methods 1400 and 1500 illustrated in the process flow diagrams of FIGS. 14 and 15, as well as variants thereof. Other embodiments include network node apparatuses configured to carry out one or more of these methods. In some embodiments of the invention, processing circuits, such as the processing module 40 and memory circuit 46 of FIG. 9, are configured to carry out one or more of the techniques described in detail above. Likewise, other embodiments may include network nodes that include one or more such processing circuits. In some cases, these processing circuits are configured with appropriate program code, stored in one or more suitable memory devices, to implement one or more of the techniques described herein. Of course, it will be appreciated that not all of the steps of these techniques are necessarily performed in a single microprocessor or even in a single module.

It will further be appreciated that various aspects of the above-described embodiments can be understood as being carried out by functional “modules” corresponding to the method steps illustrated in FIGS. 14 and 15. These functional modules may be program instructions executing on an appropriate processor circuit, hard-coded digital circuitry and/or analog circuitry, or appropriate combinations thereof, e.g., in network nodes having hardware configurations like that shown in FIG. 9.

For example, FIG. 16 illustrates an example functional module or circuit architecture as may be implemented in base station 10, e.g., based on the processing module 40 and memory circuit 46, of a first RAN operating according to a first RAT. The illustrated embodiment at least functionally includes a receiving module 1602 for receiving a handover request for a user equipment from a cell in a second RAN operating according to a second RAT. The implementation also includes a configuration module 1604 for, after handover of the user equipment to a cell in the first RAN is completed, configuring the user equipment to measure one or more frequencies corresponding to the second RAN. The implementation includes an identifying module 1606 for, based on measurements reported by the user equipment for the one or more frequencies, identifying one or more detected cells exceeding a measurement threshold. The implementation further includes a sending module 1608 for sending a handover report towards the second RAN. The handover report includes, for at least one detected cell exceeding the measurement threshold, a physical cell identifier for the detected cell and a frequency identifier for the detected cell.

In another example, FIG. 17 illustrates an example functional module or circuit architecture as may be implemented in base station 10, e.g., based on the processing module 40 and memory circuit 46, of a first RAN operating according to a first RAT. The illustrated embodiment at least functionally includes a handover module 1702 for initiating a handover of a user equipment from a cell in the first RAN to a cell in a second RAN, operating according to a second RAT, by sending a handover-required indication towards the second RAN. The implementation also includes a receiver module 1704 for, after handover of the user equipment to the cell in the second RAN is completed, receiving a handover report from the second RAN. The handover report comprises, for at least one cell detected by the user equipment, a physical cell identifier for the detected cell and a frequency identifier for the detected cell. The implementation includes an identification module 1706 for identifying a global cell identifier for the at least one cell, based on the physical cell identifier and frequency identifier. The implementation further includes a mobility module 1708 for adjusting one or more mobility settings with respect to the cell and/or with respect to the at least one cell, in response to receiving the handover report.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. For example, although embodiments of the present invention have been described with examples that reference a communication system compliant to the 3GPP-specified LTE standards, it should be noted that the solutions presented may be equally well applicable to other networks. The specific embodiments described above should therefore be considered exemplary rather than limiting the scope of the invention. Because it is not possible, of course, to describe every conceivable combination of components or techniques, those skilled in the art will appreciate that the present invention can be implemented in other ways than those specifically set forth herein, without departing from essential characteristics of the invention. The present embodiments are thus to be considered in all respects as illustrative and not restrictive.

In the present description of various embodiments of present inventive concepts, it is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of present inventive concepts. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which present inventive concepts belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense expressly so defined herein.

When an element is referred to as being “connected”, “coupled”, “responsive”, or variants thereof to another element, it can be directly connected, coupled, or responsive to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected”, “directly coupled”, “directly responsive”, or variants thereof to another element, there are no intervening elements present. Like numbers refer to like elements throughout. Furthermore, “coupled”, “connected”, “responsive”, or variants thereof as used herein may include wirelessly coupled, connected, or responsive. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Well-known functions or constructions may not be described in detail for brevity and/or clarity. The term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that although the terms first, second, third, etc. may be used herein to describe various elements/operations, these elements/operations should not be limited by these terms. These terms are only used to distinguish one element/operation from another element/operation. Thus a first element/operation in some embodiments could be termed a second element/operation in other embodiments without departing from the teachings of present inventive concepts. The same reference numerals or the same reference designators denote the same or similar elements throughout the specification.

As used herein, the terms “comprise”, “comprising”, “comprises”, “include”, “including”, “includes”, “have”, “has”, “having”, or variants thereof are open-ended, and include one or more stated features, integers, elements, steps, components or functions but does not preclude the presence or addition of one or more other features, integers, elements, steps, components, functions or groups thereof. Furthermore, as used herein, the common abbreviation “e.g.”, which derives from the Latin phrase “exempli gratia,” may be used to introduce or specify a general example or examples of a previously mentioned item, and is not intended to be limiting of such item. The common abbreviation “i.e.”, which derives from the Latin phrase “id est,” may be used to specify a particular item from a more general recitation.

Example embodiments have been described herein, with reference to block diagrams and/or flowchart illustrations of computer-implemented methods, apparatus (systems and/or devices) and/or computer program products. It is understood that a block of the block diagrams and/or flowchart illustrations, and combinations of blocks in the block diagrams and/or flowchart illustrations, can be implemented by computer program instructions that are performed by one or more computer circuits. These computer program instructions may be provided to a processor circuit of a general purpose computer circuit, special purpose computer circuit, and/or other programmable data processing circuit to produce a machine, such that the instructions, which execute via the processor of the computer and/or other programmable data processing apparatus, transform and control transistors, values stored in memory locations, and other hardware components within such circuitry to implement the functions/acts specified in the block diagrams and/or flowchart block or blocks, and thereby create means (functionality) and/or structure for implementing the functions/acts specified in the block diagrams and/or flowchart block(s).

These computer program instructions may also be stored in a tangible computer-readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instructions which implement the functions/acts specified in the block diagrams and/or flowchart block or blocks. Accordingly, embodiments of present inventive concepts may be embodied in hardware and/or in software (including firmware, resident software, micro-code, etc.) running on a processor such as a digital signal processor, which may collectively be referred to as “circuitry,” “a module” or variants thereof.

It should also be noted that in some alternate implementations, the functions/acts noted in the blocks may occur out of the order noted in the flowcharts. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Moreover, the functionality of a given block of the flowcharts and/or block diagrams may be separated into multiple blocks and/or the functionality of two or more blocks of the flowcharts and/or block diagrams may be at least partially integrated. Finally, other blocks may be added/inserted between the blocks that are illustrated, and/or blocks/operations may be omitted without departing from the scope of inventive concepts. Moreover, although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows.

Many variations and modifications can be made to the embodiments without substantially departing from the principles of the present inventive concepts. All such variations and modifications are intended to be included herein within the scope of present inventive concepts. Accordingly, the above disclosed subject matter is to be considered illustrative, and not restrictive, and the appended examples of embodiments are intended to cover all such modifications, enhancements, and other embodiments, which fall within the spirit and scope of present inventive concepts. Thus, to the maximum extent allowed by law, the scope of present inventive concepts are to be determined by the broadest permissible interpretation of the present disclosure, and shall not be restricted or limited by the foregoing detailed description. 

What is claimed is:
 1. A method, in a network node operating in a first radio access network (RAN) according to a first radio access technology (RAT), the method comprising: receiving a handover request for a user equipment from a cell in a second RAN operating according to a second RAT; after handover of the user equipment to a cell in the first RAN is completed, configuring the user equipment to measure one or more frequencies corresponding to the second RAN; based on measurements reported by the user equipment for the one or more frequencies, identifying one or more detected cells for which measurement results exceed a measurement threshold; and sending a handover report towards the second RAN, the handover report comprising, for at least one of the identified detected cells, a physical cell identifier for the detected cell and a frequency identifier for the detected cell.
 2. The method of claim 1, further comprising receiving information identifying the one or more frequencies corresponding to the second RAN in a message associated with the handover request.
 3. The method of claim 2, wherein the information identifying the one or more frequencies comprises an Evolved Universal Terrestrial Radio Access (E-UTRA) Absolute Radio-Frequency Channel Number (EARFCN).
 4. The method of claim 2, further comprising receiving, in a message associated with the handover request, measurement information indicating, for at least one of the one or more frequencies, whether or not the user equipment should measure closed-subscriber-group (CSG) cells corresponding to the at least one of the one or more frequencies, wherein said configuring the user equipment to measure one or more frequencies corresponding to the second RAN comprises configuring the user equipment to measure CSG cells or not to measure CSG cells, according to the received measurement information.
 5. The method of claim 4, wherein the measurement information is a single indicator indicating whether or not the user equipment should measure CSG cells for all of the one or more frequencies.
 6. The method of claim 4, wherein the measurement information comprises a separate indicator for each of the one or more frequencies.
 7. The method of claim 1, further comprising receiving information identifying the measurement threshold in a message associated with the handover request.
 8. The method of claim 1, wherein the handover report includes, for at least one of the identified detected cells, a global cell identifier.
 9. The method of claim 1, further comprising including, in the handover report, a physical cell identifier and frequency identifier only for those identified detected cells for which a global cell identifier could not be derived from the measurements reported by the user equipment.
 10. The method of claim 1, wherein the first RAT is Global System for Mobile Communications (GSM), Edge Radio Access Network (GERAN), or UTRA, and the second RAT is E-UTRA.
 11. A method, in a network node operating in a first radio access network (RAN) according to a first radio access technology (RAT), the method comprising: initiating a handover of a user equipment from a cell in the first RAN to a cell in a second RAN, operating according to a second RAT, by sending a handover-required indication towards the second RAN; after handover of the user equipment to the cell in the second RAN is completed, receiving a handover report from the second RAN, the handover report comprising, for at least one cell detected by the user equipment, a physical cell identifier for the detected cell and a frequency identifier for the detected cell; identifying a global cell identifier for the at least one cell, based on the physical cell identifier and frequency identifier; and adjusting one or more mobility settings with respect to the at least one cell, in response to receiving the handover report.
 12. The method of claim 11, further comprising sending, towards the second RAN, information identifying one or more frequencies to be measured by the user equipment, in a message associated with the handover-required indication.
 13. The method of claim 12, wherein the information identifying the one or more frequencies comprises an Evolved Universal Terrestrial Radio Access (E-UTRA) Absolute Radio-Frequency Channel Number (EARFCN).
 14. The method of claim 12, further comprising sending, in a message associated with the handover-required indication, measurement information indicating, for at least one of the one or more frequencies, whether or not the user equipment should measure closed-subscriber-group (CSG) cells corresponding to the at least one of the one or more frequencies.
 15. The method of claim 14, wherein the measurement information is a single indicator indicating whether or not the user equipment should measure CSG cells for all of the one or more frequencies.
 16. The method of claim 14, wherein the measurement information comprises a separate indicator for each of the one or more frequencies.
 17. The method of claim 11, further comprising sending information identifying a measurement threshold in a message associated with the handover-required indication.
 18. The method of claim 11, wherein the handover report includes, for at least one cell detected by the user equipment, a global cell identifier.
 19. The method of claim 11, wherein the first RAT is Evolved Universal Terrestrial Radio Access (E-UTRA) and the second RAT is Global System for Mobile Communications (GSM), Edge Radio Access Network (GERAN), or UTRA.
 20. A network node apparatus adapted to operate in a first radio access network (RAN) according to a first radio access technology (RAT), the network node apparatus comprising a processing circuit configured to: receive a handover request for a user equipment from a cell in a second RAN operating according to a second RAT; after handover of the user equipment to a cell in the first RAN is completed, configure the user equipment to measure one or more frequencies corresponding to the second RAN; based on measurements reported by the user equipment for the one or more frequencies, identify one or more detected cells for which measurement results exceed a measurement threshold; and send a handover report towards the second RAN, the handover report comprising, for at least one of the identified detected cells, a physical cell identifier for the detected cell and a frequency identifier for the detected cell.
 21. The network node apparatus of claim 20, wherein the processing circuit is configured to receive information identifying the one or more frequencies corresponding to the second RAN in a message associated with the handover request.
 22. The network node apparatus of claim 21, wherein the information identifying the one or more frequencies comprises an Evolved Universal Terrestrial Radio Access (E-UTRA) Absolute Radio-Frequency Channel Number (EARFCN).
 23. The network node apparatus of claim 21, wherein the processing circuit is configured to receive, in a message associated with the handover request, measurement information indicating, for at least one of the one or more frequencies, whether or not the user equipment should measure closed-subscriber-group (CSG) cells corresponding to the at least one of the one or more frequencies, and wherein the processing circuit is adapted to configure the user equipment to measure one or more frequencies corresponding to the second RAN by configuring the user equipment to measure CSG cells or not to measure CSG cells according to the received measurement information.
 24. The network node apparatus of claim 23, wherein the measurement information is a single indicator indicating whether or not the user equipment should measure CSG cells for all of the one or more frequencies.
 25. The network node apparatus of claim 23, wherein the measurement information comprises a separate indicator for each of the one or more frequencies.
 26. The network node apparatus of claim 20, wherein the processing circuit is configured to receive information identifying the measurement threshold in a message associated with the handover request.
 27. The network node apparatus of claim 20, wherein the handover report includes, for at least one of the identified detected cells, a global cell identifier.
 28. The network node apparatus of claim 20, wherein the processing circuit is configured to include, in the handover report, a physical cell identifier and frequency identifier only for those identified detected cells for which a global cell identifier could not be derived from the measurements reported by the user equipment.
 29. The network node apparatus of claim 20, wherein the first RAT is Global System for Mobile Communications (GSM), Edge Radio Access Network (GERAN), or UTRA, and the second RAT is E-UTRA.
 30. A network node apparatus adapted to operate in a first radio access network (RAN) according to a first radio access technology (RAT), the network node apparatus comprising a processing circuit configured to: initiate a handover of a user equipment from a cell in the first RAN to a cell in a second RAN, operating according to a second RAT, by sending a handover-required indication towards the second RAN; after handover of the user equipment to the cell in the second RAN is completed, receive a handover report from the second RAN, the handover report comprising, for at least one cell detected by the user equipment, a physical cell identifier for the detected cell and a frequency identifier for the detected cell; identify a global cell identifier for the at least one cell, based on the physical cell identifier and frequency identifier; and adjust one or more mobility settings with respect to the at least one cell, in response to receiving the handover report.
 31. The network node apparatus of claim 30, wherein the processing circuit is configured to send, towards the second RAN, information identifying one or more frequencies to be measured by the user equipment, in a message associated with the handover-required indication.
 32. The network node apparatus of claim 31, wherein the information identifying the one or more frequencies comprises an Evolved Universal Terrestrial Radio Access (E-UTRA) Absolute Radio-Frequency Channel Number (EARFCN).
 33. The network node apparatus of claim 30, wherein the processing circuit is configured to send, in a message associated with the handover-required indication, measurement information indicating, for at least one of the one or more frequencies, whether or not the user equipment should measure closed-subscriber-group (CSG) cells corresponding to the at least one of the one or more frequencies.
 34. The network node apparatus of claim 33, wherein the measurement information is a single indicator indicating whether or not the user equipment should measure CSG cells for all of the one or more frequencies.
 35. The network node apparatus of claim 33, wherein the measurement information comprises a separate indicator for each of the one or more frequencies.
 36. The network node apparatus of claim 30, wherein the processing circuit is configured to send information identifying a measurement threshold in a message associated with the handover request.
 37. The network node apparatus of claim 30, wherein the handover report includes, for at least one cell detected by the user equipment, a global cell identifier.
 38. The network node apparatus of claim 30, wherein the first RAT is E-UTRA, and the second RAT is Global System for Mobile Communications (GSM), Edge Radio Access Network (GERAN), or UTRA. 